The invention relates generally to a color electronic reprographic printing system, and more particularly concerns apparatus for optimizing the contact between paper or other copy media and a photoconductive surface.
The marking engine of an electronic reprographic printing system is frequently an electrophotographic printing machine. In an electrophotographic printing machine, a photoconductive member (often a photoreceptor belt) is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is thereafter selectively exposed. Exposure of the charged photoconductive member dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is treated with toner particles and is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid or a powder material.
In the process of black and white printing, the copy sheet is advanced from an input tray to a path internal to the electrophotographic printing machine where a toner image is transferred thereto and then to an output catch tray for subsequent removal therefrom by the machine operator. In the process of multi-color printing, the copy sheet moves from an input tray through a recirculating path internal to the printing machine where a plurality of toner images is transferred thereto and then to an output catch tray for subsequent removal. With regard to multi-color printing, a gripper bar secured to a transport receives the copy sheet and transports it in a recirculating path enabling the plurality of different color images to be transferred thereto. The gripper bar grips one edge of the copy sheet and moves the sheet in a recirculating path so that accurate multi-pass color registration is achieved. In this way, magenta, cyan, yellow, and black toner images are transferred to the copy sheet in registration with one another.
As the copy paper is left exposed to its environment, humidity can cause it to pucker. Other surface irregularities may be caused by mishandling of the copy stock and duplexing. These localized deformities can create an air gap between the paper and the photoreceptor belt. Such gaps result in the poor transfer of toner from the belt to the paper, which may manifest itself in deletions or distortions of information. Flipping the paper over, or discarding the old paper and adding fresh paper offer possible solutions to this problem. However, such rotation of paper stock is inherently expensive in paper costs, labor, and down time. A means for reducing the need for operator involvement and reducing the amount of paper that is wasted is needed.
A device which applies a force against the back of a sheet in transfer and flattens it against the photoreceptor belt is one possible solution to the problem. The Xerox Corporation 5090 machine uses a device which uses four flexible Mylar.TM. blade segments, each of which is deflected back away from the photoreceptor belt by solenoid actuated mechanisms. One or more of the solenoids are activated by the passage of a sheet, depending on the paper size being used. The force applied against the sheet is a function of the deflection of the mylar blade. Since the blades of these machines are held in a deflected-back state both during standby and between each copy, the mylar may tend to take on a permanent set over time, decreasing the force applied. This may result in the degradation in performance, over time, of the blades, and the need to replace the blades frequently.
There remains a need for a device that will provide enhanced contact between a copy sheet and a photoreceptor belt that is reliable and requires little maintenance.